_A_, _B_, Blastula, or simple hollow ball; _C_, _D_, intermediate condition; _E_, _F_, gastrula, or double-walled flask condition.] But there is no known type of animal which, in its adult form, shows quite the simple structure that we have described. Perhaps the nearest approach is to be found in the lower sponges, in which two modifications of the original plan have already been introduced. In the first place, the creature is sedentary, being fixed, in an inverted position, to some solid basis. It has, so to speak, ceased to be a hunter, and is become a fisher. Secondly, its wall is pierced in many places, so as to permit of a freer circulation, through the digestive cavity, of the water which contains the food material. The water passes in through these numerous perforations, and out through the main central opening or 'mouth.' The sponges do not appear to represent a stage in the main line of Evolution, but lead us almost immediately into a cul-de-sac. We therefore cannot pause to describe fully the many peculiar and interesting developments which occurred in the group. An ordinary 'sponge,' by the way, bears the same relation to the creature which produces it as does a 'coral' to the coral animal. It represents, that is to say, the skeletons of a large colony of individuals. The structure of a sponge is shown in Fig. 32. [Illustration: FIG. 32.--Diagrammatic section of lower sponge. _e_, inner cell layer. _m_, middle jelly-like layer. _z_, outer cell layer. _a_, digestive cavity. _i_, perforations in the wall.]